1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a laser beam printer, that has the function to transfer a toner image onto a sheet (a recording material).
2. Description of the Related Art
In conventional image forming apparatuses, such as copying machines and laser beam printers, using electrophotographic techniques, a print (printing) position relative to a recording material may deviate due to the accuracy of conveyance of the recording material, the variance of the conveyance accuracy, or the accuracy of position of a toner image to be printed. Therefore, there have been proposed various techniques regarding the alignment (registration) of a toner image relative to a recording material.
For example, Japanese Patent Application Laid-Open No. 60-120369 discusses the technique to temporarily stop a registration roller pair at a point of time when the recording material reaches a detection sensor and then re-start driving the registration roller pair with precise timing to perform registration.
Also, Japanese Patent Application Laid-Open No. 3-36559 discusses the technique to arrange a recording-material detection sensor in the conveyance path for a recording material, form an electrostatic latent image with precise timing based on a detection signal from the recording-material detection sensor, and transfer a toner image, formed by developing the electrostatic latent image, onto the recording material.
In addition, Japanese Patent Application Laid-Open No. 2003-248410 discusses the technique to detect the leading end position and lateral end position of a recording material by using an image sensor, calculate the amount of positional deviation based on the detected leading end position and lateral end position, and correct the writing start timing of an electrostatic latent image in the sub scanning direction and the writing start position in the main scanning direction.
However, the above-mentioned techniques to perform alignment between a recording material and a toner image have the following issues to deal with. Specifically, the conventional techniques, while being directed to the alignment between a recording material and a toner image, perform correction based not on the position detection at a portion where the toner image is transferred onto the recording material but on the position detection at a portion located upstream of that portion. Thus, the conventional techniques perform prediction-based correction and, therefore, may leave, as an actual transfer positional deviation, the sum of the amount of deviation of a toner image during the process from the formation of an electrostatic latent image to the arrival at the transfer position onto a recording material and the amount of deviation of the recording material during the process from the detection position for the recording material to the arrival at the transfer position onto the recording material.
Furthermore, to reduce any positional deviation, an installation engineer for the apparatus may be required, during installation of the apparatus, to repeat test printing and perform visual confirmation and adjustment of a print position relative to the recording material. Also, in some cases, a user may be required to regularly perform confirmation and adjustment of the print position.
More precise adjustment for alignment between a recording material and a toner image may require a test print with a plurality of sheets. Since the print position relative to a recording material may have variance, if a test print is performed with a single sheet, the print position may be greatly affected by such variance. Therefore, it is desirable to perform a test print with a plurality of sheets, for example, ten sheets, to confirm the print position for each printed sheet.
Furthermore, the print position may slightly vary depending on types of recording material, for example, plain paper, coated paper, and recycled paper. In addition, when the moisture content of paper changes depending on the environment where the recording material is placed, the expansion or contraction of the recording material may cause a difference in print position. These issues may affect printing accuracy. Also, a recording material that has been heated and pressed when passing through a fixing device, for example, during two-sided printing, expands or contracts due to a great change in moisture content. This issue may also affect printing accuracy. To deal with these conditions, such a cumbersome and complicated operation as to confirm the accuracy of a print position in each condition or mode may be required.